AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Making a Transistor Radio – Part 1
N
early 50 years ago I tried
to build a radio. I was young,
inexperienced… and it didn’t
work. I wasn’t the only one to try. In fact,
it must be one of the most popular radio
projects ever printed, but the strange thing
is it didn’t appear in any of the regular
radio or electronics magazines – it wasn’t
even aimed at adults. It appeared in a
massively popular and influential series
of children’s books called ‘Ladybird’. (If
you have a little time to spare then there
is a very pleasant (non-electronic) hour
to be spent watching this documentary:
http://bit.ly/pe-mar21-lb).
48 years to get it working
All these years later that early failure to
finish the project bugged me, and this
project will set that right! In fact, not
only will we build the complete original
version of the Ladybird Radio, but also
I have a few upgrades to suggest. First
though, a little background.
The idiosyncratic serious-amateur culture here in the UK makes the country’s
art and technology output surpass its economic size. Maybe this was because it was
the cradle of the Industrial Revolution,
but I suspect the plethora of electronics
magazines and books were a major factor.
Among these, were the Ladybird series of
children’s illustrated books, and some of
my collection is shown in Fig.1.
Life begins and ends
I suspect for many of us, an engineering
or science career really starts around
the age of ten. For me, that was 1972,
the year the Reverend George Dobbs’
Ladybird Book Making a Transistor Radio came out – shown in Fig.2. It even
helped me learn to read, since nothing
else at school captured my interest.
This book entered the Children’s Top
Ten for a while and influenced a generation of youngsters. Unfortunately, I
never got to meet George Dobbs. I was
hoping to interview him about his book
at the 2018 Welsh Radio Rally at Newport in October 2018, but he was ill and
cancelled his lecture. He died shortly
afterwards aged 75 in March 2019. (Practical Wireless magazine, however, did
interview him in their June 2009 issue.)
Old shops
Luckily, I found that one of my component suppliers, John Birkett’s of Lincoln,
had known George as a family friend
for more than 50 years. John’s shop was
established in 1962, and is one of the
last bricks-and-mortar stores remaining,
along with a few others, such as Cricklewood Electronics and JPR. He still has
some of George’s books and supplies all
the old components for those wishing
to build the Ladybird Radio and vintage
electronic equipment. John is now in his
nineties, so today the shop is mainly run
by his daughter Judy, who incidentally
was christened by Rev Dobbs. There is
an interview with them both in the Jan
2020 issue of Practical Wireless.
Finding copies today
Fig.1. A small sample of the famous
Ladybird books.
66
Making a Transistor Radio has long been
out of print, but you might strike lucky
at a boot fair or charity shop and pay
50p. Of course, eBayers may have got
there before you. On the Net they can
cost up to £20 for a clean early edition,
but the average cost is around £5-8, even
though they often say ‘15p’ on the back!
A little Googling will probably enable
you to find an online version.
Fig.2. Making a Transistor Radio, an
influence on many of PE’s older readers?
Look Mum! – No Computer!
There were many brilliant beginner’s technology books in the Ladybird series. The
books were ‘The Internet’ of my childhood.
They introduced my ‘plastic’ mind to all
forms of technology.
I migrated from the Ladybird books to
Hamlyn Publishing’s colour paperbacks,
because they were visual rather than the
algebraic nature of most technical books.
Electronics, written by Roland Worcester
(a pseudonym of FG Rayer who wrote
many articles for 1970s electronic magazines) was a favourite. It had fantastic
colour circuit diagrams, and since no
one collects these books they are going
for junk prices (I give them to students).
Next, I started soldering at age 11, sat on
the floor and wiping my Antex iron on
my mother’s white carpet. (A mistake I
was advised to make only once!)
Electronic art
Just as in botany and anatomy books, the
concepts and construction in the Ladybird and Hamlyn series were explained
Practical Electronics | March | 2021
n
Fig.3. Painted illlustrations can often explain technical
detail better than a photo (Isabella Rothman).
with illustrations – not photographs.
These are clearer than photos because
the main points can be emphasised and
redundant information omitted. One
of my neighbours was an illustrator for
the Ladybird Peter and Jane series, sadly she didn’t do the radio illustrations,
that was Bernard Robinson – see: http://
bit.ly/pe-mar21-lb1
Bernard was my favourite Ladybird
illustrators, and he was especially interested in music and technology. We can’t
use the Ladybird Book illustrations here
because Penguin owns the copyright,
but my daughter Isabella has drawn a
similar example in Fig.3. Long may she
continue the tradition!
Real breadboarding
Even in the ‘Supersonic Seventies’, the
publishers considered soldering to be too
dangerous for children, so George came
up with a unique construction system using brass screws and cups. This involved
clamping the wires down on a piece of
wood to make the connections, resulting in a genuine breadboard, as shown
in Fig.4. There was one problem with
this technique, you had to be physically strong enough to do the woodwork.
I got nowhere on my first attempts because I used hardwood – for a child to
drive screws into wood it has to be a softwood, such as pine. Drilling recesses for
pot nuts with a brace-and-bit was also
difficult; I went all the way through on
my first attempt. If only I’d had a Makita
power screwdriver/drill in those days!
2mm pilot holes are much better than a
bradawl for starting screws.
I then asked my mother to buy me the
Ladybird woodwork and metalwork books
from the Manchester bookshop where she
worked. In turn, I have used these books
to teach the same skills to my children.
The strange thing is, after a day of employing the screw-and-cup technique I
got used to it. I now use it in areas of solderless prototyping, such as connecting
large components to modern breadboards
and passive loudspeaker crossovers. The
screws do need periodic re-tightening,
however. Also, a combination of wires of
different diameters under the same cup
can result in the thinnest wire, such as
from a transistor coming loose.
in Fig.5. With the large surface area of the
Ladybird ‘breadboard/screw’ technique
it gets much lower. Fortunately, germanium transistor circuits are generally low
impedance, so most (non-treated) wood
is fine. I used 9mm plywood.
Today, I suspect a new children’s construction technique would use ‘choc-blocks’.
However, that would lack the unique visual
clarity of the Ladybird method.
Getting it done… eventually
I’m sad to say my first Ladybird Radio never worked. It didn’t put me off
though, because I just knew electronics
was what I wanted to do. I did get the
wooden multivibrator working in Dobbs’
second Ladybird book, Learn about...
Simple Electronics. I still use this circuit
to introduce electronics to my students.
I know we all have ‘skeleton projects in
our closets’; but this one, at 48-years late
was my worst, so I decided to have another go at the radio to see what went wrong.
Was it ‘bad’ wood – or me? I started with
the book’s first circuit, the crystal set.
Conductive wood
Fig.4. George Dobbs’ screw-and-cup
connection technique.
Practical Electronics | March | 2021
When we get to the construction stage,
beware of slightly green-tinged pieces of
wood used for fencing and decking. These
are tanalised to prevent rot, a process
that impregnates the wood with copper
compounds, even arsenic was used in
the US until 2003. This means the wood
is useless for ‘bread-boarding’ because it
becomes mildly electrically conductive
when damp. The resistance can drop to
about 250kΩ between probes, as shown
Fig.5. Tanalised wood can be conductive
– do not use it in this project.
67
Fig.6. The simplest radio – a crystal set, and the first design in the Ladybird book. Note the OA70 glass diode, centre right.
Crystal set
The simplest radio construction is a crystal set, shown in Fig.6. It is completely
passive and needs no battery, using the
radio wave energy itself. Of course, crystal
sets only receive ‘old-fashioned’ amplitude modulated (AM) transmissions, such
as BBC Radio 4 on 198kHz. I live near a
medium-wave transmitter by the Ithon
river in Llandrindod Wells in Mid-Wales,
which provides a strong signal with an
external aerial. (I must try a frame aerial
wound on an umbrella as in the Japanese
Kasa crystal set: www.kasaradio.com/en/
index.html). Unfortunately, all I could get
inside the workshop was switch-mode
power supply buzz, so polluted has our
RF world become. Even outside, all I got
was Radio 5 Live. I suspect the crystal set
has now become an unusable historical
object. I also suspect human hearing sensitivity has declined due to traffic noise and
phone use. With the output of the crystal
Current
flow
Modulated radio
frequency
*OA70 or
galena crystal
Detector
diode*
Electromagnetic
radiation
from
transmitter
190µH
50 turns
Tap
(not
used)
500pF
Tuned circuit output
voltage peaks at
resonant frequency
Fig.7. The Ladybird crystal set circuit.
68
set in the order of microwatts, most people today just can’t hear it.
The circuit shown in Fig.7 couldn’t be
simpler. All it needs is a parallel tuned
circuit and a detector to demodulate the
waveform. This is achieved by rectifying
the signal; the resulting DC level reflects
the modulation or envelope.
To obtain maximum power to feed the
crystal set, a long high aerial and a good
earth are necessary. If either of these are
insufficient then the set won’t work at all.
Remember, there is no chance of a ferrite rod alone picking up enough signal
for a crystal set – you really do need a
long, well-insulated conductor and a
good earth to complete the circuit. As
a child, I used my bed’s metal frame for
the aerial. This time, I upgraded to a
washing line made with wire and a copper water pipe for the earth, as shown
in Fig.8. Another upgrade worth trying
is making the aerial’s suspension from
glass insulators (Fig.9).
Audio frequency
High-impedance
crystal earpiece
from the 1950s. However, its piezoelectric
transducer has high sensitivity and high
impedance. So much so that I could hear
a click just touching one lead on the ring
on my finger (the other held in my other hand). Annoyingly, I couldn’t get the
thing to stay in my ear without sliding a
bit of pink silicone sleeving over it. Also,
I found I was completely deaf to very low
sound levels in my right ear.
Apparently, there is a more modern type
that uses piezoelectric ceramic discs. These
transducers also act as a smoothing capacitor
for the demodulator, having a capacitance
of around 15nF. I also tried a pair of Beyer
Headphones
The headphones
used should be 4k
high-impedance balanced-armature types,
which have excellent
sensitivity. These are
very hard to obtain today. A crystal earpiece
(Fig.10) can also be used,
which is the next best
thing and easier to buy.
It’s an ugly component,
looking like a hearing-aid
Fig.8. A good earth is needed for a
crystal set aerial. Copper water pipes
going into the ground are ideal. (Watch
out, blue plastic Alkathene pipes are
often used for water mains today). Put a
steel reinforcing rod into the copper earth
pipe if you are going to bang it into hard
soil. I used a radiator water pipe as a kid.
Practical Electronics | March | 2021
Fig.9. A good aerial is vital. A horizontal
loop around the picture rail in a Victorian
house is good enough to get the crystal
set working, but for best results string it up
outside as high and long as possible using
glass insulators. I found this insulator on
the side of a disused railway!
Dynamic DT100 moving-coil studio headphones of 2kΩ impedance and they were
surprisingly insensitive and not much use.
Taken prisoner
Crystal sets have a fascinating history
as the basis for POW (prisoner of war)
radios. Most were based on the crystal
set and bits stolen from old telephones.
The only ‘real’ electronic component
required was a telephone earpiece, and
POW versions of these were possible if
you could find magnets and fine copper
wire. The tuning capacitor could be a
pair of tin cans sliding in and out. The
detector was often a galena crystal, basically lead ore or lead sulphide, a natural
semiconductor. This mineral, shown in
Fig.11, can be found on the ground in my
home county of Derbyshire; my friends
and I all tried making crystal sets with it.
George Dobbs has a chapter on POW
radios in his book and recommended using a piece of coke; I had no luck with
this when I tried a lump from the school
playground coke heap. (This was before
natural gas heating and every school had
a pile of coke fuel).
These old detectors were ‘point-contact’
devices, precursors to the transistor. The
Fig.10. Crystal earpiece – they look awful and sound worse, but do respond to just a few mV.
point of contact was critical and much poking about was required to get a rectifying
crystal boundary. A sharp graphite pencil or springy wire filed to a point works
well; oxidised wire does not work at all.
Once a suitable spot is found, it has to be
held exactly and firmly in position. The
wire can be wound into a spring as shown
in Fig.12. Interestingly, I found the galena
crystal shown in Fig.11 had a very low resistance of around 4Ω when probed on new
shiny surfaces. Being a natural mineral, its
electrical properties are highly variable.
Any particular sample will have different
contaminants. The resultant doping action
can change the semiconductor from P to N
type and sometimes just to a conductor. It
is possible to buy selected ‘radio-grade’ galena crystals. These are generally mounted
in a pot of Wood’s metal. All crystals can
only exhibit diode action at certain contact
positions, such as oxidised areas or crystal boundaries. The diode action is often
very poor. Measuring the forward voltage
drop on a multimeter diode-test function I
had 200mV one way and 300mV the other.
This provides a degree of rectification and
the high reverse leakage stops the capacitive crystal earpiece getting fully charged
up, negating the need for a load resistor.
Real diodes
To get round all this hassle, the germanium small-signal diode was developed.
These are the only point-contact semiconductor devices still made. They were
Fig.11. Galena crystals (right) combined
with quartz – an electronic hippy’s dream.
Practical Electronics | March | 2021
Fig.12. A detector using a galena crystal
based on the Ladybird book design.
Note the springy pointed wire, sometimes
known as a ‘cat’s whisker’.
also one of the first semiconductor devices to be mass produced, necessary for
the Second World War effort. They are
usually encapsulated in glass, allowing
the crystal and point-contact wire to be
clearly seen, as shown in Fig.13. This
type was an early STC device. Similar
looking OA70s were used in the circuit
shown in Fig.6 and Fig.7. Old Siemens
OA85 diodes are still available, but they
are coated in black paint to stop light increasing the leakage current. The Ladybird
design used the OA81 which was very
popular in 1960s UK radios. All these big
old diodes have the SO-15 case.
There are a few physically smaller types
available, such as the OA91, 1N34A and
CG92 which will do the job. The OA91
is the OA81 in a DO-7 case. If you can’t
get a germanium diode, a Schottky type
such as an IN60, ZC5800 or BAT42 can
work almost as well in crystal sets. After
all, it could be argued a galena detector,
with its metal-semiconductor junction is
a form of Schottky diode. Normal silicon
diodes such as the 1N4001 or 1N4148
won’t work because of their high forward
voltage and sharp knee, although 0.5V
of forward bias from a battery will get
it to work. The Kasa radio site suggests
using an LED and exposing it to light to
generate its own bias voltage.
Next month
In the next part we’ll get to the heart of
the book and build a true transistor radio.
Fig.13. An old germanium point-contact
diode. Note the slab of germanium and
the spring-loaded point-contact wire.
69
